1. Field of the Invention
The present invention relates to substrates fabricated from a reinforced polymeric material, which can be metallized by a reduction treatment followed, if appropriate, by electrolytic metallization, which metallized substrates are well adopted for the manufacture of printed circuits. The invention also relates to such metallized substrates, per se, ofttimes referred to simply as "metal-clad".
The present invention also relates to a process for the production of said substrates.
2. Description of the Prior Art
The metallized substrates are themselves well known to this art (cf. U.S. Pat. No. 4,110,147). Typically they comprise an electrically insulating support material, to either or both of the face surfaces of which is adhered a conductive metal sheet. This metal sheet may be in particular a sheet or foil of copper, aluminum, nickel, or stainless steel having a thickness ranging from 10 to 100 micrometers, depending upon the type of printed circuit sought to be produced.
The metallized substrates under consideration may be rigid, semi-rigid or flexible, depending upon the composition of the insulating support material. By "semi-rigid substrate" there is intended a material which can withstand an elastic deformation by bending same to a very small radius of curvature.
In the case of the rigid or semi-rigid metallized substrates, to which the present invention more particularly relates, the insulating support material is usually formed by the stacking of a number of prepregs, each of which comprises the combination, per se known to this art, of elongated reinforcing filler material within a matrix of a polymeric material. With a conventional reinforcing filler such as, for example, a glass cloth weighing 200 g/m.sup.2 or an impregnated paper sheet, 6 to 12 prepregs are employed, on average. The conventional prepregs consist of cellulosic papers, cotton cloths or glass cloths impregnated with synthetic polymers. Phenol-formaldehyde resins, polyester resins and, especially, epoxide resins are the materials employed most frequently. The reinforcing filler, paper or glass cloth, is generally impregnated with a solution of polymer in a suitable solvent, which permits good penetration of the polymeric binder between and among the filler fibers. The impregnated structure then is placed in an oven heated to a temperature which causes the solvent to evaporate. This technique is usually designated the "collodion route".
The manufacture of metallized substrates includes inserting, between the pressure plates of a press, the stack of prepregs which is covered with a metal sheet on one or both of the face surfaces thereof, depending on whether it is desired to produce a mono- or bimetallized substrate. Consolidation by pressing is then carried out a temperature which permits lamination of the various components. In certain cases, the metal sheets require an adhesive interlayer to be permanently bonded to the prepregs.
For obvious economic reasons, it is desirable to increase manufacturing output by reducing the number of components, in particular the number of prepregs, which constitute the product laminate. This simplification at the manufacturing level should of course not be made at the expense of the mechanical and electrical properties of the final product.
It is also a desideratum in this art to produce metallizable and metallized substrates, the manufacture of which does not give rise to any environmental pollution. As heretofore mentioned, the preparation of the insulating support material typically entails a process for impregnating a series of reinforcing structures using a solution of polymer in a suitable solvent. In order to produce a prepreg which can be employed in the downstream operations, the solvent must be removed by drying. This removal of solvent, despite all of the precautions taken to recover the same, is frequently a cause of pollution. Other disadvantages which may be mentioned, which are also associated with the use of solvent are, on the one hand, its purchase price and, on the other, the large quantity of energy needed for its drying. Reduction in the number of prepregs, mentioned earlier, to simplify the manufacture of metallized substrates, thus, still appears to be one way of solving this pollution problem. Another way to solve or avoid this problem completely would be to abandon the collodion route for the preparation of the remaining prepregs.